The observation has been made that an isolated tracheal smooth muscle preparation from the rat, if exposed to high concentration of isoproterenol at frequent intervals, will eventually become insensitive to the relaxing action of the drug (Fleisch and Titus, 1972). This finding has been confirmed in our laboratory. In the research proposal presented in this application we have described experimental procedures which would extend this initial observation. An attempt will be made to achieve four broad objectives. They are: (1) to characterize the mechanism by which the tracheal smooth muscle becomes less sensitive to the relaxing action of isoproterenol. This will include studies to estimate the concentrations of isoproterenol and the durations of exposure of the tissue to isoproterenol required to produce various levels of desensitization, to examine the possibility that the reduced affinity of the Beta-receptor for propranolol, which we have already observed to occur after prolonged exposure of the tracheal muscle to isoproterenol, may provide an adequate explanation of the underlying basis of desensitization, and to further examine our preliminary finding that a change in the affinity of the Beta-receptor may be brought about by the chemical reduction of disulfide groups to sulfhydryl groups; (2) to determine whether the type of desensitization of tracheal smooth muscle, which was found to occur in vitro, will also occur in vivo; (3) to investigate a number of experimental conditions and pharmacological agents for their possible modifying effects on the desensitization of tracheal smooth muscle to isoproterenol. The influence of temperature and catecholamine uptake into the tissue as well as the influence of a variety of spasmolytic agents such as aminophylline, nitroglycerine and Beta 2 agonists will be considered in this investigation; (4) to determine whether or not cardiac muscle can also be desensitized to the chronotropic and ionotropic actions of isoproterenol, and if so, to characterize this desensitization process.